Distributed Feedback Interband Cascade Laser Based Laser Heterodyne Radiometer for Column Density of HDO and CH4 Measurements at Dunhuang, Northwest of China
Round 1
Reviewer 1 Report
The atmospheric HDO and CH4 density in Dunhuang, China were measured by Laser heterodyne radiometer. This paper introduces the algorithm and fitting process of spectral inversion in detail. This paper compares the difference between the measurement results and those of other instruments and discusses the possible reasons. The work is worth publishing, but the manuscript itself needs to be revised. The author needs to answer or modify the following questions before the manuscript can be considered for publication.
- It is suggested to give the complete word of LHR in the appropriate position of the manuscript, followed by LHR.
- How do “Jacobian value and the Averaging Kernels” contribute to detection sensitivity and accuracy, or do they play other roles?
- For the fitting in Figure 6, giving the fitting contour of individual absorption peaks is necessary.
- The methane density in Fig.7 b is obviously around 1.792, while in line 184, the value 1.786± 0.01 is reported. Another question is how about the ± 0.01 was determined, does the uncertainty be overestimated?
- In line 189, “ the following figure” is not proper.
- In line 266, Line intensity has no units.
Author Response
We are grateful for your positive comments and the suggestions, the suggestions are very helpful to improve the presentation of the paper. We answer the suggestions one by one sincerely according to the comments and have checked the manuscript and revised them, more details are given in the attached document. Furthermore, the recommendations are valuable and helpful for our future improvements, thank you very much!
Author Response File: 
author_response.pdf
Reviewer 2 Report
The manuscript focuses on the measurements of atmospheric column density of HDO and CH4 using laser heterodyne radiometer which is the second contribution work from the same group, the authors characterized the ILS of the self-build radiometer, and use the ILS to retrieve the column and profile density of the HDO and CH4. The retrieved densities were then compared with the densities measured by another commercial available instrument--EM27/SUN. The results are interesting and helpful for the related researchers, the LHR based measurements will find many meaningful applications, and therefore I think the manuscript could be accepted with minor revision:
1. The data obtained from the field measurements is not adequate enough, only several days, I would suggest the authors presenting more data if possible.
2. It is interesting that the LHR and EM27/SUN measure the gases through the same path, therefore the measured density should be the same, however, the difference between them seems a little bit large, although the authors present several possible reasons to explain the gaps, but there is still a point that has not been explained clearly, that is, the variation range of LHR measurement results is greater than EM27, it will be better to give more description.
3. The authors show the comparison results for water vapor, but the comparison results for methane are missing, it’s better to show the results for both gases.
4. The wavelength accuracy and repetition of the local oscillator are very important parameters for LHR system; more information should be given to illustrate these parameters.
5. When measuring the gas concentration, it is better to select the transition which is free of interference of other gases. There is no relevant analysis in the manuscript.
6. The latest publication about CH4 detection should discussed. [Optics Express. 30(2), 1304–1313 (2022)].
7. The author used heterodyne radiometer for the measurement, but other important methods are missing. I suggest the author to add them. Such as photoacoustic spectroscopy [Photoacoustics 25 (2022) 100330; Optics Letters 47(3), 601–604 (2022)] and light-induced thermoelastic spectroscopy [Optics Express. 2021, 29, 23213-23224; Chinese Optics Letters. 2022, 20(3), 031201].
Author Response
We are grateful for your positive comments and the suggestions, the suggestions are very helpful to improve the presentation of the paper. We answer the suggestions one by one sincerely according to the comments and have checked the manuscript and revised them, more details are given in the attached document. Furthermore, the recommendations are valuable and helpful for our future improvements, thank you very much!
Author Response File: author_response.pdf
Reviewer 3 Report
The paper presents the systematic research of LHR, includes the structure, instrumental line shape (ILS) of LHR, the retrieval method for HDO and CH4 and the analysis of the retrieved densities. The progresses and results are interesting for the LHR researches. Therefore, I recommend to accept the paper for publication after minor revision.
- The ILS is one of the most important parameters for spectrometer/radiometer, the simulated ILS of LHR is given in this paper, the measured ILS shoud be supplemented.
- The layers setting of the retrieval is “account the characteristic of atmospheric water distribution”, the description is too simple, the details should be presented.
- The atmospheric HDO column density is retrieved by absorption spectra in 3.53μm, but the abundance of HDO is more significant for atmospheric circulation or environmental research, it will be better to retrieve HDO abundance.
- The retrieved water vapor densities of LHR have been compared with EM27/SUN, the density of CH4 could be also obtained by EM27/SUN, the comparison of CH4 densities should be presented.
Author Response
We are grateful for the positive comments and the suggestions of the three anonymous reviewers, the suggestions are very helpful to improve the presentation of the paper. We answer the suggestions one by one sincerely according to the comments and have checked the manuscript and revised them, more details are given in the attached document. Furthermore, the recommendations are valuable and helpful for our future improvements, thank you very much!
Author Response File: author_response.pdf
